In methods of treating an organic waste water mentioned above, one extensively used method subjects a biomass sludge predominantly containing microbial cells, as well as an excess sludge containing an untreated residual sludge, to a solid-liquid separation using a precipitation tank or the like. These sludges have been produced by the biological digestion of the waste water. In this method, the resulting liquid portion obtained as a supernatant from such a separation process is appropriately discarded, and the excess sludge portion is disposed of at sea or in a landfill.
Applicant previously filed a patent application (Japanese Laid-Open Patent Publication No. 9-10791) directed to a method and an apparatus for treating an activated sludge comprising the steps of: (i) subjecting an organic waste water to an aeration treatment in an aeration device; (ii) solid-liquid separation into a treated liquid and a sludge; (iii) returning a portion of the separated sludge to the aeration device through a recycle route; (iv) heat-exchanging the excess sludge obtained from the solid-liquid separation step using a heat exchanger; (v) solubilizing the heated sludge at a high temperature in a solubilization device; and (vi) returning the solubilized solution to the aeration device through a return route. This patent application discloses an invention directed to a method and an apparatus for treating an activated sludge which is capable of drastically reducing the quantity of excess sludge produced. A schematic flow diagram for this method is shown in FIG. 4.
However, depending on the raw organic effluent that is treated, a large amount of phosphorus components (e.g., orthophosphoric acids, polyphosphoric acids, phosphate salts and their esters, phosphoproteins, glycerophosphoric acids, phospholipids, and the like) can remain in the treated liquid and the excess sludge by the method described above. Discarding such substances can directly result in environmental pollution. In particular, it is undesirable to discharge a treated liquid containing a large amount of such phosphorus components into lakes or ponds, which can cause a drastic growth of phytoplankton associated with the eutrophication of water. Therefore, a procedure used at times comprises adding a flocculant to the treated liquid obtained from the solid-liquid separation device in order to reduce the amount of the phosphorus components, followed by discharging the treated liquid. However, this method has disadvantages because subjecting a large amount of the treated liquid to such a flocculation process requires a large-scale device, thereby causing an increase in cost, time, labor, and the like, which is necessary to practice this method. Furthermore, flocculation efficiency is low, and removal of the phosphorus components at times is insufficient. Therefore, discarding an excess sludge without sufficiently removing the phosphorus components contained in the sludge is not desirable.
Meanwhile, conventional processes for removing phosphorus components from a waste water include, for example: (1) chemical flocculation method; (2) a crystallization dephosphorizing method; and (3) an anaerobic-aerobic activated sludge method (see, Sewage Service Project, Design Guide and Review, published by Japan Sewage Work Association, Vol. 2, pp.131-136, 1994).
In the chemical flocculation method, flocculants, such as aluminum sulfate, are mixed with a waste water to precipitate flocs of insoluble phosphate salts (including a floc of microorganisms). Precipitation is based on the fact that trivalent metal cations, such as aluminum ion and ferric ion, chemically react with orthophosphoric ions to form insoluble phosphate salts. According to this method, an increase of excess sludge of 5% to 20% has been reported. Therefore, in order to preserve the environment, it is not desirable to dispose of a large amount of excess sludge containing a large amount of phosphorus components.
The crystallization method is based on the production of an insoluble hydroxyapatite by a reaction between orthophosphoric ions and calcium ions. This method is preferred because an increase in excess sludge does not occur. However, it is necessary to strictly control the conditions required for crystallization of apatite (e.g., removal of crystallization inhibitor, such as carbonate ion, and pH adjustment, temperature adjustment, as pretreatments). Therefore, the applicability of this method is limited. Furthermore, because the method includes factors causing an increase in cost, it is not a preferred method of waste water treatment.
In the anaerobic-aerobic activated sludge method, an effluent is subjected to a repeated treatment in an anaerobic tank, an aerobic tank, and a precipitation tank. Thus, phosphorus components can be present in the excess sludge, and thereby the phosphorus components in the treated liquid can be reduced. This method is based on the phenomenon that microorganisms in the anaerobic state release polyphosphoric acid in the form of orthophosphoric acid, although microorganisms ingest and metabolize an excess amount of orthophosphoric acid in the aerobic state in order to accumulate orthophosphoric acid as polyphosphoric acid. According to this method, although the phosphorus components can be effectively removed from the treated liquid, the excess sludge is rich in phosphorus components and also contains various other organic components and heavy metal components. Therefore, discarding such an excess sludge caused problems. Furthermore, despite a possibility of effective uses of the phosphorus components present in the waste water, e.g., in the production of a fertilizer, phosphorus compounds, and the like, there are disadvantages in discarding the phosphorus components in the form of a sludge mixed with such heterogeneous components.